In non-self-luminous display devices such as liquid crystal display devices, so called backlight devices that illuminate display panels from behind are generally provided. These backlight devices divide into a direct-lit type, a side-lit type, a planar light source type, etc., and the side-lit type is widely adopted as display devices are made slimmer and lighter.
In a side-lit backlight device, light from a light source is led into a light guide plate through a side surface thereof and is made to travel inside the light guide plate by total reflection; in addition, light is partly reflected on a reflective sheet fitted on a back surface of the light guide plate; thus light emerges through a main surface of the light guide plate to act as a planar light source, to thereby illuminate a back surface of a display panel. As light sources, cold-cathode tubes as a linear light source have conventionally been used. However, with increasing consideration to the environment these days, LEDs (light emitting diodes) as a point light source, have come to be increasingly used.
FIG. 6A is a schematic view showing how an LED is fitted in a conventional liquid crystal display device. With the liquid crystal display device shown in FIG. 6A taken up as an example, the structure of the conventional liquid crystal display device will now be described. The liquid crystal display device is provided with a backlight device 2′ and a liquid crystal panel 1. In the backlight device 2′, a box-shaped lower chassis 21b open at the top and bottom faces thereof is fitted with a back sheet metal 26 so as to cover the bottom-face opening; on the back sheet metal 26, a box-shaped light guide plate 22 is fitted.
On the inner wall of the lower chassis 21b that faces the side surface of the light guide plate 22, an FPC (flexible printed circuit board) 31—having a plurality of LEDs (point light sources) 32 mounted thereon in the length direction thereof—is fixed with double-faced adhesive b, with the FPC 31 housed inside a reflective case C having a square-cornered rectangular section with an opening part thereof facing the side surface of the light guide plate 22. An upper chassis 21a having an opening part formed therein to let pass light emerging from the light guide plate 22 is so fitted as to cover the top-face opening of the lower chassis 21b; by the upper chassis 21a and the lower chassis 21b, the back sheet metal 26, the reflective sheet 24, the light guide plate 22, and optical sheets 25 are held together.
On a step part 28 formed around the rim of the opening part of the upper chassis 21a of the backlight device 2′, a peripheral part of the liquid crystal panel 1 is placed; a bezel 5 is placed on top, from above. Fixing the bezel 5 and the upper chassis 21a to each other completes the liquid crystal display device in which the liquid crystal panel 1 and the backlight device 2 are put together.
In the liquid crystal display device structured as described above, the FPC 31 is fitted to the reflective case C with the double-faced adhesive b. Thus, in a high temperature environment for example, the adhesive strength of the double-faced adhesive b may lower, causing the FPC 31 to come off.
For another example, as shown in FIG. 6B, conventionally, an LED 32 is mounted on a printed circuit board 51 formed of glass epoxy or the like, and this printed circuit board 51 is fixed to a lower chassis 21b with double-faced adhesive b. However, as in the example described above, the adhesive strength of the double-faced adhesive b may lower in a high temperature environment. Moreover, the thermal conductivity of the printed circuit board 51 is relatively low, and thus heat accumulates in the LED 32, causing diminished light emission efficiency and a shortened lifetime of the LED 32.
Thus, in cases where a high-power LED is used, as shown in FIG. 6C, a metal-core circuit board 52 with high thermal conductivity is used as a circuit board having an LED 32 mounted thereon, and that metal-core circuit board 52 is fitted to a lower chassis 21b with a screw S.
As described above, with the LED 32 mounted on the metal-core circuit board 52, and with this circuit board 52 screwed to the lower chassis 21b in the backlight device 2′, it possible to prevent the circuit board 52 from coming off and to prevent the LED 32 from having a shortened lifetime due to heat accumulated in the LED 32 through the circuit board 52 etc. The metal-core circuit board 52, however, is significantly expensive compared with an FPC or the like when used as a circuit board.
In a backlight device disclosed in Patent Document 1, a circuit board having an LED mounted thereon is fixed to a heat dissipation plate with fitting screws; when the circuit board is a flexible one, such as an FPC, the circuit board warps in a high temperature environment, decreasing the area of contact with the heat dissipation plate. When the area where the circuit board and the heat dissipation plate are in contact with each other is decreased, the heat generated in the LED is not adequately dissipated, possibly leading to diminished light emission efficiency and a shortened lifetime of the LED.    Patent Document 1: JP-A-2006-49098 Publication (paragraph number 0089, FIG. 5, etc.)    Patent Document 2: JP-A-2003-281924 Publication    Patent Document 3: JP-A-2006-11242 Publication